Method of making a flexible plastic tube

ABSTRACT

A flexible plastic tube, comprises, an extruded inner tube or tubular portion and an extruded outer tube or tubular portion of a thermoplastic material overlying the inner tubular portion and spaced radially outwardly therefrom and including a reinforcing strip of substantially U-shape configuration wound around the inner portion and bonded to the inner and outer tubular portions. The reinforcing strip has radially extending sidewalls in the form of fins which define a first cavity therebetween and, in addition, successive turns of the reinforcing strip define a second cavity therebetween which is located between the outer and inner walls. The sidewalls advantageously have radially outwardly tapered cross-sectional shapes. The inner wall is first extruded and may advantageously be reinforced by embedding axially reinforcing fibers in its outer surface and, while it is still not set, the reinforcing strips of a harder material in the inner and outer walls are wound around the inner wall so that it becomes bonded to the inner wall as the inner wall is set. Thereafter, the outer wall is extruded and bonded immediately to the leg portions of the reinforcing strip. In addition, a metalized material may be positioned by alignment of grooves into the first cavity between the sidewalls of the reinforcing strip and applied under tension during the application of the outer wall.

This is a division of application Ser. No. 906,757 filed May 17, 1978,now U.S. Pat. No. 4,172,474.

FIELD AND BACKGROUND OF THE INVENTION

This invention relates to the construction of flexible plastic tubes ingeneral and, in particular, to a new and useful flexible plastic tubeand a method of making the same which includes a reinforcing strip woundaround an inner tubular portion and bonded thereto and having side legswhich extend radially outwardly from each side of the reinforcing stripwhich is bonded into the outer wall.

DESCRIPTION OF THE PRIOR ART

In a tube of the above-mentioned type, known from GermanOffenlegungsschrift No. 2,261,126, a wire is helically wound directly onthe inner tube and is accommodated in a helical cavity which is formedbetween the inner tube and outer tube, and has a small axial extensionas compared to the winding pitch of the wire. A band of a relativelyhard, thermoplastic or elastomeric material is wound between the wireturns and parallel thereto. The inner tube, the band, and the outer tubeare firmly joined to each other, while the wire helix is freely movablewithin the helical cavity.

To ensure this mobility of the wire, the wire must be made of a materialwhich does not enter into any mechanical junction with the materials ofthe inner and outer tubes and of the band. Inner tube, outer tube, andband are made of polyvinyl chloride, referred to as PVC in the followingdescription, while the helical wire is made of a polyamide, polyolefine,or even of metal, see German Offenlegungsschrift No. 2,261,126.

In this plastic tube of the prior art, the wire helix is the solestabilizing element. The band, which is wound between the turns of thewire helix and does not extend laterally quite up to the wire, turns,and exclusively serves the purpose of preventing the inner tube, in thezones between the wire turns, from forming inwardly directed folds whenthe tube is bent. It is also made of a soft, and therefore, elasticmaterial. Thus, the purpose of the band is to ensure, notwithstandingthe excellent strength of the tube, that even upon bending, the insidesurface remains as smooth as possible, without affecting the highflexibility of the tube. With this prior art tube, however, the problemis only partially solved, because as the tube is bent, inwardly directedfolds of the inner tube are necessarily formed in the zones of the innerbending radius where the wire turns are applied.

In addition, the contour of this known tube is markedly undulated, whichcannot be considered suitable for every use. Further, this prior arttube is distinctly prone to buckling upon being bent to a small radiusand there is also a risk that the outer tube will separate from the bandor the inner tube, since during the bending of the tube, the radialtensional forces which the wire turns have to withstand, and which theytransmit, directly, or indirectly through an intermediate layer, in theradially outward direction, act on the outer tube which, in addition, isacted upon, in the zone of the outer bending radius, by axial tensionalforces. These simultaneously occurring tensions may cause a separationof the outer tube from the band or the inner tube. This again entails adanger that upon a strong bending, the inner tube will collapse to aflat shape, which, in the extreme, may reduce the cross-sectional areaof passage to zero.

It should be further noted that in its simplest construction, this knowntube comprises at least four different elements, namely, the inner tube,the wire helix, the band and the outer tube, all of which must beassembled in a definite operational sequence which is well coordinatedin time and space. To this end, it is first provided to form the innertube by spraying or extruding a thick layer of PVC of about 0.5 mm on amandrel and then to wind the wire helically on the inner tube, in afirst winding station. Subsequently, in a second winding station, theband of hard PVC is wound, on which the outer tube is finally extruded.

Aside from a number of other plastic tubes, which in some instances areformed of spirally or helically wound, overlapping strips with embeddedsupporting spirals and flat reinforcing bands wound over the joints,(see U.S. Pat. No. 2,798,508), a flexible plastic tube with a helix andaxial reinforcing threads is known from German Utility Model No.6,937,013, in which the smooth inner wall is formed of a cylindrical,extruded inner tube supporting the axial reinforcing threads over whichthe helix is extruded or wound, and the outside is formed of an outertube extruded on and snugly enclosing the helix and the threads.

In all of these known plastic tubes, the wire or supporting helix isused to provide the tube with a radial stability which, at the sametime, should not disadvantageously affect the flexibility of the tube.Without taking into account the formation of inwardly directed bendingfolds which are caused during the bending by the supporting or wirespirals and may sometimes reduce the cross-sectional area of passage,the manufacture of such tubes is complicated and expensive since, ineach instance, a plurality of winding elements must be applied, as faras possible, simultaneously, and partly in a state suitable for heatsealing, and united with each other. For this purpose, expensive andcomplicated winding mechanisms in multiple execution are required.

In another known plastic tube (Swiss Pat. No. 405,836), both the outerand the inner tubes are formed of foil bands which are heat-sealed toeach other at their edges and are heat-sealed together between thehelically extending turns of a reinforcing insert. No empty space(filled only with air) extending from turn to turn is provided betweenthe turns of the reinforcing insert.

The crucial importance of a helical cavity between the individual turnsof the reinforcing insert or band for the flexibility of the tube and ofthe reinforcing band for the formation of folds, during the bending ofthe tube, is already well known from, for example, GermanOffenlegungsschrift No. 2,261,126. Tubes without such a cavity ofnecessity have a substantially reduced flexibility and less favorableconditions for forming folds during bending. The mentioned prior arttubes of Swiss Pat. No. 405,836 and German Offenlegungsschrift No.2,261,126 are vacuum cleaner tubes which, as compared to a standardwater hose, for example, have a relatively large inner diameter, forexample, of 1.5 inch=38 mm. With such thick tubes, not only is theirouter undulated configuration less conspicuous, but also a smallerflexibility is expected relative to thinner tubes, such as water hoses,which have an inner diameter of 1 inch or less. While with vacuumcleaner tubes, the flexibility thereof is considered satisfactory if thetube allows a bending radius of 10-15 cm, with water hoses, particularlyshower hoses, for example, a much higher flexibility is expected,namely, one which permits a bending radius of from 2.5 to 3 cm.

While in the prior art tube of German Offenlegungsschrift No. 2,261,126,the radial stability depends only on the wire helix, in the known tubeof Swiss Pat. No. 405,836, this stability results from thecross-sectional shape of the reinforcing insert, which incidentally,attaches more importance to the capability of floating of this insertthan to the cross-sectional shape thereof. Only the occluded airincreasing the capability of floating is considered important.

As is well known, T-sections, U-sections and double T-sections, whenmade of a hard or stiff material, are particularly resistant to bending,due to the tension-to-thrust ratio between the outer and inner bendingradii. For this reason, such T-sections, double T-sections andU-sections are unsuitable as supporting helices or reinforcing bands fortubes having a small diameter, for example, of 1 inch or less. That is,with a small tube diameter and, consequently, small winding radius, thezones of the outer bending radius are subjected to a very strong tensionand the zones at the inner bending radius to an equally strongcompression, so that folds are formed on the inside and elongations inexcess may occur in the zone of the outer radius, completely frustratingthe supporting effect which is just based on the resistance to bending.Experience has also shown that when being wound on a tube of smalldiameter, radially outwardly projecting legs of a conventional channelsection fold flat to the inside or outside, so that the radial stabilityor the air occlusion to improve the floating capability which is desiredcannot be obtained.

SUMMARY OF THE INVENTION

The present invention is directed to a flexible plastic tube which ismore stable in the radial direction and more flexible and is easier tomanufacture, and comprises an inner tube which, upon being bent withinthe range of its own flexibility, does not form bending folds along theinner bending radius and remains, at least substantially, smooth.

To this end, in accordance with the invention, a flexible plastic tubeis provided in which the helical cavity is empty and bounded laterally,in the axial direction, not only by the narrow sides of a reinforcingstrip, but also by continuous reinforcing fins which extend in thelongitudinal direction of the strip at each narrow side thereof, whileforming another cavity. The fins have a cross-section tapering radiallyoutwardly which diverge symmetrically relative to the median radialplane of the strip, so that the cross-sectional area of the cavity,formed between the fins and the strip, tapers radially outwardly.

Due to the inventive cross-sectional shape of the reinforcing fins ofthe reinforcing strip, the above-mentioned drawbacks and unfavorableproperties for the helical winding of conventional reinforcing sectionsof the prior art are avoided. The inventive reinforcing strip sectionmay be easily wound on an inner tube having a relatively small outsidediameter of about 10 mm. In addition, the surprising effect is therebyobtained that the bending stress produced in the reinforcing fins by thewinding, imparts to the tube, a much greater radial stability than thatwhich would be expected in view of the standard resistance to bending ofthe shaped reinforcing strip.

Due to the presence of the empty cavity between the individual turns ofthe reinforcing strip and to the particular shape of the cross-sectionthereof, it is possible for the first time to provide a tube which, witha small inside diameter, is not only extremely flexible, but alsoextremely stable in the radial direction and, at the same time, providesparticularly favorable conditions for the folding of the inner tubeduring bending. Aside from this, it is of particular advantage that thisis obtained without an additional wire helix, i.e., only with astabilizing element in the form of a reinforcing strip which can bewound onto the inner tube during the extrusion thereof. In addition, thereinforcing strip is best suited to be provided on its outside with ametal foil or metal coating which gives the tube a metalic aspect ingeneral. Thus, the inventive tube is superior to tubes of the prior artnot only in its specific properties of flexibility, radial stability andfolding capability during bending, but also in its simple manufactureand is, consequently, inexpensive.

The reinforcing fins also very substantially contribute to flexibilityof the tube. The crests of the reinforcing fins may be bonded to theouter tube extruded thereon. Because of their relatively thin walls, thefins are laterally flexible, i.e., during the bending, and depending onwhether they therefore extend into the zone of the inner or outerradius, they may move toward each other, where the outer and inner tubesare compressed, or move away from each other, where the outer and innertubes are stretched. Consequently, during the bending of the tube, thewidths of the helical intermediate spaces vary, with the result thatboth an extreme folding at the inner bending radius and a flattening ofthe tube in the zone of the outer radius are prevented. This is furthersupported by the fact that alternately extensible and compressiblecavities are present not only between the individual turns of thereinforcing strip, but also between the reinforcing fins of thereinforcing strip and the outer tube, and that the cross-section ofthese cavities can vary during the bending.

In a specific embodiment of the invention, it is provided that thereinforcing fins are offset on the outside of the reinforcing strip fromthe longitudinal edges thereof toward the center, by approximately 1/3or 1/4 of the width of the strip. In a preferred embodiment, however,the reinforcing fins are provided along the two helically extendinglongitudinal edges of the reinforcing strip since, in such anembodiment, the outside surface of the reinforcing strip, which isvisible through the transparent outer tube, and also the surface of thereinforcing fins facing each other, may be covered with a metal strip ora metalized plastic strip in a very simple manner, to give the finishedtube a metalic aspect.

It has proven to be advantageous to design each reinforcing fin with anangle of divergence, in the stretched state of the reinforcing strip, ofabout 15° to 25°.

According to a development of the invention, two groove-like recesses,which are separated from each other by an intermediate land, areprovided on the outside of the reinforcing strip, between thereinforcing fins. These groove-like recesses may also be designated asgrooves. The groove-like recesses extend directly adjacent thereinforcing fins, and are advantageous in many respects. With a giventhickness of the reinforcing strip, they increase the radial height ofthe reinforcing fins, for example, reinforcing threads may be insertedin these groove-like recesses.

Another advantage of the groove-like recesses is obtained where it isprovided that the inside surfaces of the reinforcing fins and/or theoutside surface of the reinforcing strip extending therebetween arecovered with a metal foil, for example, aluminum foil, or with a plastictape coated with a metal layer, for example, an aluminum layer depositedby evaporation. Such metal foils or metalized plastic tapes have beenknown for a long time in connection with plastic tubes of this kind.Their sole purpose is to give the tube a metalic appearance.

In a known plastic tube of the kind set forth in the preamble of thebase claim, this purpose is served by a flat reinforcing band in which ametalized, for example, an aluminum, vapor-coated, polyester strip isembedded and, in addition, underlaid with textile, for example, rayonthreads. This means that the metalic appearance is produced during theextrusion of the reinforcing band, by inserting the aluminum coatedpolyester strip into the band while the latter is formed by extrusion.Depending on the thickness of the polyester strip, which is flexible,but not elastic, and due to its strength, it provides the reinforcementproper of the tube, thereby, quasi representing the reinforcing banditself, so that the polyester strip must be completely enclosed in a PVCcovering, to permit heat-sealing of the reinforcing band to both theouter tube portion and the inner tube portion, with the polyester stripitself being embedded in the PVC covering of the reinforcing bandwithout material integration.

As compared to this complicated manufacturing method, thecross-sectional shape of the inventive reinforcing strip makes itpossible to apply a metal foil or metalized plastic tape of suitablewidth in a substantially simple way, by winding the tape onto thereinforcing strip either simultaneously with the winding of the latteron the inner tube, or, which may be done subsequently, during theextrusion of the outer tube on the reinforcing strip which has alreadybeen joined to the inner tube. Since, during the winding of the metallicor metalized tape, which must be done under a certain tension, to ensurea satisfactory smoothness of the surface, the tape adapts to thecross-sectional shape of the reinforcing strip, the two groove-likerecesses have the effect of stabilizing the position of the wound tape.

In another design of the invention, an undercut groove may be providedin each of the surfaces of the reinforcing fins facing each other,adjacent the strip surface extending therebetween.

Accordingly, it is an object of the invention to provide a flexibleplastic tube which comprises an inner tube or tubular portion of arelatively soft plastic material, and a substantially U-shapereinforcing strip of relatively hard plastic, and having a generallyradially extending thin sidewall adjacent each side thereof and beinghelically wound around the inner tubular portion and bonded thereto, andhaving an axially spaced apart turn around said inner tubular portion,and including an outer tube or tubular portion covering the inner tubeportion and the reinforcing strip, and being bonded to the outer ends ofthe thin walls of the reinforcing strip and, wherein, the outer wallencloses a first cavity between the thin walls and the inner surface ofthe reinforcing strip and a second cavity between adjacent turns of thereinforcing strip and said outer wall and said inner wall.

A further object of the invention is to provide a method of making aplastic tube, comprising, extruding an inner tubular portion, applying areinforcing strip of substantially U-shape cross-section over the innertubular portion while the inner tubular portion has still not set sothat the reinforcing strip becomes bonded to the inner tubular portion,and thereafter, applying an outer tubular portion by extrusion over thereinforcing strip and bonding it to the reinforcing strip.

Another object of the invention is to provide a flexible plastic tubewhich is simple in design, rugged in construction and economical tomanufacture.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Drawings:

FIG. 1 is a partial perspective view indicating the formation of aplastic tube, constructed in accordance with the invention;

FIG. 2 is a cross-sectional view of another embodiment of thereinforcing strip;

FIG. 3 is a view, similar to FIG. 2, of still another embodiment of theinvention;

FIG. 3a is a section, similar to FIG. 2, of yet another embodiment ofthe invention;

FIG. 4 is a view, similar to FIG. 2, showing another embodiment ofreinforcing strip, with a metalized coating thereon;

FIG. 5 is a partial longitudinal sectional view of a complete tubeconstructed in accordance with the invention; and

FIG. 6 is a partial longitudinal, sectional view of a tube constructedin accordance with the invention, indicating the bending thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings in particular, the invention embodied thereinas shown in particular in FIGS. 1, 5 and 6, comprises, a composite tubewhich is made up of an inner tube or tubular portion of a soft material,such as PVC, a substantially U-shaped reinforcing strip 3 of a harderplastic material, and an outer tube or tubular portion of soft PVCmaterial, with the reinforcing strip 3 being bonded to both the innerand outer tubular portions 2 and 5.

FIG. 1 shows a portion of a semi-finished plastic tube 1, in which aU-shape open-topped reinforcing strip 3 is helically wound around theouter surface of an inner tube 2 of soft PVC, with the enclosure ofaxially extending reinforcing threads 4 which are placed directly on theouter surface of inner tube 2. As may be seen in FIGS. 5 and 6, afinished plastic tube further comprises an outer tube or tubular portion5 which is extruded onto the reinforcing strip. FIGS. 2, 3, 3a and 4show different cross-sectional shapes of the reinforcing strip 3, 3",31, respectively, of which the design of FIG. 4 has proven to be mostadvantageous.

In the embodiment of FIG. 3a, reinforcing strip 31 comprises tworeinforcing fins 8" which diverge from each other and are each offsetfrom the edge of the strip toward the center by about 1/4 to 1/3 of thestrip width. These reinforcing fins 8" are inclined outwardly, each byan angle of divergence α of about 15° and their function is at leastsubstantially equal to that of reinforcing fins 8 of reinforcing strip 3shown in FIGS. 4 and 5, which will be explained in more detailhereinafter.

It is common to the cross-sectional shapes of FIGS. 2, 3 and 4, that oneach of its two longitudinal sides, reinforcing strip 3 is provided withcontinuous, longitudinally extending and readially outwardly projectingreinforcing fins indicated at 6, 7 and 8 for FIGS. 2, 3 and 4,respectively. While reinforcing fins 6 have a cross-sectional shape of arectangle beveled on the inside, the cross-sectional shape ofreinforcing fins 7 resembles a right-angled triangle. In addition, ontheir insides adjacent the outer surface 3' of the body proper 3" of thestrip, reinforcing fins 7 are provided with undercut grooves 9 intowhich a metal foil or metalized plastic tape 17, as shown in FIG. 5, forexample, may be inserted.

In both designs, it is important that the beveled or inclined surfacesof fins 6 and 7 are provided at their facing sides and that they divergeradially outwardly. This is necessary to ensure that, during the windingof reinforcing strip 3 onto inner tube 2 which is formed on a mandrel,the reinforcing fins 6 and 7 do not tilt toward each other, but awayfrom each other, quasi outwardly, as shown, for example, in FIG. 5.

The preferred cross-sectional shape of FIG. 4 differs from those ofFIGS. 2 and 3 in several details. First, it may be noted that in thestretched state of strip 3, each of the two reinforcing fins 8 isinclined outwardly by an angle of divergence α, and that a groove-likerecess 10 is provided at the base of each reinforcing fin 8, with thetwo recesses being separated from each other by an intermediate land 11.The inside surfaces 12 of reinforcing fins 8 are convex and blendsmoothly into recesses 10. In the stretched state of reinforcing strip3, the angles of divergence α amount to about 20°, and upon windingreinforcing strip 3 onto inner tube 2, they increase to about 25°.Reinforcing strip 3 of the embodiment of FIG. 4 is provided with twoclaw ribs 14 and 15 of triangular cross-section on its plane underside13. While both the inner tube 2 and the later applied outer tube 5 aremade of a soft PVC material, the reinforcing strip 3 is made of a hardPVC material.

The axial reinforcing threads 4 and reinforcing strip 3 are applied toinner tube 2 directly at the extruder head forming the inner tube,before inner tube 2 cools down, so that a heat-sealed bond isimmediately established between reinforcing strip 3 and inner tube 2. Atthe same time, the claw ribs 14 and 15 are passed over a heated tube sothat they are also plasticized, and axial threads 4 can penetratetherein, as shown in FIG. 5. The claw ribs 14 and 15, in turn, penetrateinto the outer surface of inner tube 2. Thereby, axial threads 4 arewell anchored to the turns of reinforcing strip 3, and the reinforcingstrip 3 is well anchored to inner tube 2.

During the operation of winding reinforcing strip 3 onto the inner tube2, further reinforcing threads 16 are passed into the grooves orrecesses 10 and, at the same time, a metalized plastic tape 17 is woundover the reinforcing threads. The tape 17 is fed in under tension andoccupies the position, and assumes the shape, shown in FIGS. 4 and 5.The width of the metalized plastic tape is selected so as to cover notonly the area between adjacent reinforcing fins 8, 8 of reinforcingstrip 3, but also the convex inner surfaces 12 of the reinforcing fins8, except for the portions of the fins 8 which become embedded in, andheat-sealed to, the outer tube 5 during the extrusion thereof. In thisway, the entire surface of reinforcing strip 3, visible from the outsidethrough outer tube 5, is covered with the metalized plastic tape.

In FIGS. 4 and 5, it may be noted that the metalized plastic tape, whichis fed in under tension, forms depressions 18 in the area of groove-likerecesses 10. These depressions 18 substantially contribute to thelateral positional stability of the metalized plastic tape 17 onreinforcing strip 3. This positional stability is of importanceparticularly if an intermediate storage, for example, winding up of thesemifinished tube, is necessary or provided between the operation ofapplying the reinforcing strip 3 to inner tube 2 and the extrusion ofouter tube 5. This stabilization of position, however, is advantageousalso in instances where the operation of winding reinforcing strip 3onto inner tube 2 is immediately followed by the extrusion of outer tube5.

In order to obtain radial stability, it is important to wind thereinforcing strip onto inner tube 2 under a definite tension. A smallwinding tension results in a smaller radial stability, while a strongertension results in a higher radial stability. This also applies to thecross-sectional shapes of FIGS. 2, 3 and 3a.

It may be learned from FIG. 6, that cavities 19 and 20 are formedbetween the adjacent turns of reinforcing strip 3 and also betweenadjacent reinforcing fins 8 of each turn of the reinforcing strip 3. Theouter ends of reinforcing fins 8 partially penetrate into the wall ofouter tube 5 and are integrally joined thereto.

The cavities 19 and 20 make the finished tube 1 very flexible, despitethe very high radial stability of the tube. This flexibility is due tothe fact that the inner tube 2 and outer tube 5, which are made of asoft PVC material, are relatively elastic, and the reinforcing fins 8are bendable in the axial direction of the tube both toward and awayfrom each other, so that cavities 19 and 20 may enlarge or contract, asthe need arises. In the zones at the outer bending radius R1, cavities19 and 20 are enlarged in the axial direction and reduced in the radialdirection, i.e., a linear extension within the elasticity limits takesplace.

On the contrary, in the zones at the inner bending radius R2, cavities19 and 20 lessen in width, due to the pushing together of the turns ofreinforcing strip 3, and also due to the movement against each other ofthe respective reinforcing gins 8 belonging to the same turn. At thesame time, in this zone, outer tube 5 assumes an undulated shape, aradially inwardly extending wave trough forming between reinforcing fins8 belonging to the same turn, and an outwardly extending wave crestforming in the zone of cavities 19.

To ensure this flexibility or bending elasticity of tube 1, even incases where the tube is provided with axial reinforcing threads 4, thenumber of which may amount to ten or twelve, it is provided that whilethe axial reinforcing threads 4 are brought in position on inner tube 2,loops are formed in the zones between every two turns of reinforcingstrip 3, which permits the tube to stretch in the zone of outer bendingradius R1. This also confers to the tube a certain special elasticitydetermined by the inner and outer tubes. Such a plastic tube may also beemployed as a high-pressure tube, in which case, however, thereinforcing strip 3 must be provided in addition with reinforcingthreads 16 extending in the groove-like recesses 10.

It should be added that in view of the positional stabilizationdescribed in the foregoing, there is no need for an integral bondbetween the metalized plastic tapes 17 and the reinforcing strip 3.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A method of forming a composite plastic tube,comprising, extruding an inner tubular wall portion, winding acontinuous length of a U-shaped reinforcing strip around the innertubular wall portion in successive spiral turns which are axially spacedapart, said reinforcing strip of U-shape configuration having side wallswhich define a first cavity therebetween over said reinforcing strip anda second cavity between sidewalls of adjacent strips, said side wallseach having a radially diverging outwardly tapering cross-sectionalshape symmetrically diverging from the radial median plane of saidstrip, by which the cross-sectional shape of said second cavity istapered in the radially outward direction, and applying an outer tubularwall by extruding it over the reinforcing strip so as to join it to theside walls of said reinforcing strip.
 2. A method of forming a compositeplastic tube, as claimed in claim 1, including applying a metal intosaid first cavity between said side walls of said reinforcing stripunder tension before the outer wall is applied thereto.